Plasma flow in force-free magnetospheres: two-fluid model near pulsars and black holes

TL;DR

This paper develops a two-fluid model for force-free magnetospheres near pulsars and black holes, linking particle dynamics to electromagnetic fields and enabling analytic modeling of complex astrophysical environments.

Contribution

It introduces a novel method to incorporate particle behavior into force-free solutions, extending the framework to curved spacetime and providing analytic models for magnetospheres.

Findings

Derived a general perturbative method for magnetosphere modeling

Produced analytic models for pulsar and black hole magnetospheres

Showed potential to replace large-scale simulations with analytic approaches

Abstract

Force-free electrodynamics describes the electromagnetic field of the magnetically dominated plasma found near pulsars and active black holes, but gives no information about the underlying particles that ultimately produce the observable emission. Working in the two-fluid approximation, we show how particles can be "painted on" to a force-free solution as a function of boundary conditions that encode the particle output of "gap regions" where the force-free approximation does not hold. These boundary conditions also determine the leading parallel electric field in the entire magnetosphere. Our treatment holds in a general (possibly curved) spacetime and is phrased in language intrinsic to the 1+1 dimensional "field sheet spacetimes" experienced by particles stuck to magnetic field lines. Besides the new results, this provides an elegant formulation of some standard equations; for example, we show that the zero-gyroradius guiding center approximation is just the Lorentz force law on the field sheet. We derive a general perturbative method and apply it to pulsar and black hole magnetospheres with radial magnetic fields to produce fully analytic models that capture key features of the full problem. When applied to more realistic magnetic field configurations together with simulation-informed boundary conditions for the gap regions, this approach has the potential to provide global magnetosphere models without the need for global particle-in-cell simulations.